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How I fit Boxster calipers and vented rotors to my '66 912

My '66 912 was a basket case when I bought it as a project many years ago, it still has most of the original parts (except for the engine), but was rusty and in need of lots of work. I tore it apart and started rebuilding it about 20 years ago, but it ended up in the back corner of the shop for a while gathering dust as life got in the way.

I recently started working on it again and realized it's never going to be a show car, but it will make a fun Outlaw/hot rod project. I'm doing an engine swap and wanted larger, better brakes that will be able to deal with the added weight, power, and (hopefully) speed.

Since it still had the original early suspension with the thin solid rotors, cast iron calipers, and postage stamp size pads, I decided to upgrade to vented rotors front and rear, and go with Boxster aluminum 4 piston monobloc calipers, they are a popular upgrade, light, strong, easy to find, inexpensive, and Brembo supplied. They are an easy bolt on for later 911's using aftermarket adapter brackets with the 3.5" caliper bolt spacing on the struts, but since the early 911's and 912's have a bolt on bracket with a 3" caliper bolt spacing, none of the aftermarket adapter brackets fit - so I had to make my own.

I could have removed the orignal suspension and bolted on the suspension from a later 911, but since I had already stripped, blasted, painted, and rebuilt the original suspension I chose to keep it and see if it was possible to improve the brakes without scrapping everything and starting over, and I wanted to to keep from having to make any irreversible changes or modifications - I wanted to keep the upgrades simple, bolt-on parts changes, so it would be possible to replace the original parts if I ever wanted to in the future. Upgrading to vented rotors was easy - I removed the old solid rotors from the hubs and bolted on 24mm wide vented rotors from an '85 Carrera.

A local guy parts out wrecked Boxsters, I was able to pick up a set of calipers in good shape and several mounting bolts for a good price. I bought some steel plate, some aluminum bar stock, and got to work.

The factory front caliper bracket bolts to the strut but is too large to work with the Boxster calipers.


I used 3/8" thick steel plate and fabricated a bracket to allow bolting on an adapter to work with the Boxster calipers' radial mount bolt pattern. I cut out the large center hole for the spindle with a 2" hole saw, then drilled the mounting bolt pattern, drilled and tapped the outobard holes for the adapater brackets, and relieved the area around the spindle hole with a carbide burr bit in a router to give room for the back of the hub.

(Sorry for the huge pics, the board resizes them no matter how I load them)





The adapter brackets are simply 1"x2" solid 6061 T6 aluminum bar stock, drilled through one side for the bracket bolts, and drilled and tapped on the other side for the 12mmx1.5 bolts with the 130mm caliper bolt spacing.




I used the extra old caliper bolts to test fit the adapter bracket to the strut bracket, will have to get correct length bolts in the future, but they work well for this prototype mockup.




The calipers look upside down in some of the photos because I hadn't gotten around to swapping the fluid transfer line and bleeders yet.



A 15" rim clears the outside of the caliper with room to spare, but hub spacers will be needed to move the backside of the spokes away from the face of the caliper.

Last edited by earlybroncoguy; 02-21-2018 at 05:06 AM..
Old 02-20-2018, 10:16 AM
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Nice! I like it!
The pre 68 hub is a 40 mm as I recall. I have a 67. And the 68 is a 47 mm hub? If that's true you could bolt on a pair of 68 and maybe later hubs and forgo the spacers.
Chris
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Last edited by cmpski; 02-22-2018 at 06:31 AM..
Old 02-20-2018, 03:05 PM
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Very nice!

What size spacer to clear the rim?
Old 02-22-2018, 04:45 AM
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Like cmpski said you can use a different hub and you will not need to use a spacer.
See this post.
Can anyone confirm: front wheel hub depth '67 vs later
Old 02-22-2018, 09:13 AM
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What are you doing for the rear brakes ?
Old 02-22-2018, 09:17 AM
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If I do use a spacer to clear the calipers, it won't be very thick, probably 1/8" -1/4". I may try the later front hubs and not need spacers.

As far the rear, I'm still working on those. The rotors are easy, '85 944 rear rotors are vented but otherwise nearly identical to early 911 solid rear rotors, only difference is the hole for the retaining screw needs to be drilled in a slightly different location.

Getting the Boxster calipers mounted to the early steel trailing arms with the tiny 2.25" caliper mounting bolt pattern is tough - the ears on the trailing arm are right where the caliper needs to be. The top ear is in the way no matter where I try to "clock" the caliper, I may end up having to notch the bottom of the inner brake pad slightly to allow the caliper to fit over the rotor correctly. I will be able to use the bottom threaded ear on the trailering arm to bolt the aluminum bar caliper adapter to, but I may end up having to drill and tap the trailing arm casting to bolt the top of the caliper bracket to. I don't really want to modify original steel SWB trailing arms, but I don't see any other way to do it (yet).
Old 02-22-2018, 02:48 PM
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If you find a solution for the rear, please post it. There have been some solutions but I have not seen any using Boxster calipers.
Old 02-22-2018, 06:55 PM
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TRE does or did offer a SWB Boxster caliper bracket.
Chris

TRE Motorsports
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1959 Auratium Green 356A Super w/ Rudge wheels
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Old 02-23-2018, 05:07 AM
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Quote:
Originally Posted by cmpski View Post
TRE does or did offer a SWB Boxster caliper bracket.
Chris

TRE Motorsports
If they did, they don't anymore - or at least it's not on their website. The one on the website is only for the later 3.5" caliper bolt spacing struts, which is what all the other aftermarket adapters I have seen are.

I'm not sure why no one offers adapters for the early cars with the 3.0" caliper bolt spacing, unless it's because it's not possible to use the orginal bracket that bolts to the strut - that's the problem I ran into, which is why I had fabricate my own. At least it turned out to be relatively easy, just a flat plate that bolts to the strut and provides a flat surface to bolt the caliper adapter to - no bends, curves, or offest was needed.

I used 3/8" steel plate, 1/2" would have been too thick (back side of rotor would hit it), 7/16" would work but that thickness of steel plate is very hard to find.
Old 02-23-2018, 05:48 AM
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Quote:
Originally Posted by earlybroncoguy View Post
If they did, they don't anymore - or at least it's not on their website. The one on the website is only for the later 3.5" caliper bolt spacing struts, which is what all the other aftermarket adapters I have seen are.

I'm not sure why no one offers adapters for the early cars with the 3.0" caliper bolt spacing, unless it's because it's not possible to use the orginal bracket that bolts to the strut - that's the problem I ran into, which is why I had fabricate my own. At least it turned out to be relatively easy, just a flat plate that bolts to the strut and provides a flat surface to bolt the caliper adapter to - no bends, curves, or offest was needed.

I used 3/8" steel plate, 1/2" would have been too thick (back side of rotor would hit it), 7/16" would work but that thickness of steel plate is very hard to find.

If you have not joined-I would highly suggest that you join the 912 bbs forum, it's free and very responsive and active from a lot of 912'ers--just a highly suggested comment--yes it's free and always introduce yourself and your car make, model and year and you will be off running driving this beauty!
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Old 02-24-2018, 05:16 AM
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For the rear did you check how PMB is doing this ? They were selling larger rear calipers with an adapter similar to the one you are using in the front. This is what you are trying to do but as you are saying there is a problem.Most probably the problem you are having is that the Boxster calipers are much larger than the ones used by PMB.
Enclosed a picture (I took it from the PMB site) of the calipers that PMB is selling with the adapter.

Old 02-26-2018, 01:54 PM
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earlybroncoguy,
did you find a solution for the rear brakes ?
Old 05-23-2018, 12:30 PM
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OK, here is the solution I've come up with to mount Boxster radial mount rear calipers to SWB steel rear control arms. I did not want to modify my original '66 912 SWB control arms, but I did end up having to drill and tap one new 10mm hole above the large hole in each arm that the parking brake cables pass through, and had to remove one of the sheet metal ears on each backing plate that the dust cover bolts to (not a big deal, since the dust covers won't fit with the huge Boxster calipers anyway).

The biggest challenge was the original upper caliper bolt hole in the control arm, it occupies the space where the bottom center of the inboard pad of the Boxster caliper ends up, and is not in a usable location for mounting the new caliper adapter bracket. Also, the area of the control arms where the original caliper bolt holes are located has a slight recess cut into it for the original calipers, I had to fabricate a small piece of filler plate out of steel to fill in that area and make the backside of the control arm flat for the adapter bracket to bolt to.

The adapter brackets are 6061 T6 aluminum, 6 3/8" long, 1.5" x 1.5" cross section, drilled through for the 10mm control arm mount bolts and the 12mm top caliper bolt, and drilled and tapped for the lower caliper mount stud (the bottom of the adapter brackets sit so close to the control arms that a stud is needed.)







I also removed the original 40mm long wheel studs and pressed in longer, 60mm studs after slightly enlarging the diameter of the holes in the hub for the new studs' larger knurl diameter. The heads of the new studs are taller than the original studs, they are close to the parking brake shoe retaining springs, but clear them by about 2mm.






In the photos above and below, you can see the original top caliper mounting hole in the control arm is too close to the caliper body to be useable for the adapter bracket, which is why I had to drill and tap a new hole in the control arm a little further back. I tried clocking the caliper further down to clear that original bolt hole and be able to use for the adapter bracket, but the caliper body is so large the lower inboard corner interferes with the control arm if moved that far, this is as far as it will go and still clear. The adapter bracket blocks the hole that the parking brake cable passes through, I still need to drill a hole in the bracket for the cable.



You can see how tight the clearance is at the bottom of the adapter bracket, the lower inboard corner had to be notched to clear the control arm. It doesn't touch, but it's very close. This is why a stud had to be used for the bottom caliper mount - no room for a bolt head on the backside of the adapter bracket. I also decided threading a stud into the aluminum block and leaving it there was better than removing and reinserting a steel bolt into aluminum threads every time the caliper need to come off.




Rotors are stock '85 944 rear vented rotors, they fit the '66 912 stub axle hubs almost perfectly, the outer diameter of the hubs only needed a slight trim for the inner hat of the rotors to fit. They were nice and shiny when I took them out of the box from Rock Auto, but with my constant handling and Texas humidity, flash rust has set in with a vengeance.





Stock Boxster pads fit the 944 rotors perfectly. The bottom center of the inboard pad had to be notched slightly to clear the casting protrusion of the control arm bolt hole, but that's it. I cut and removed the stock Boxster pad wear sensor wiring from the pads, since obviously I won't be using them on a '66 912.


Last edited by earlybroncoguy; 08-27-2018 at 07:07 AM..
Old 08-26-2018, 06:54 PM
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interesting.
is all this DIY adaptation street legal in the US?
just curious, as for such there is no chance for TÜV in Germany.
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Old 08-28-2018, 10:57 PM
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Thanks for sharing
Old 08-29-2018, 12:21 PM
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Son of a gun, after all the work I put into fabricating the rear brackets, thinking that no one else had ever done anything like that, I came across this thread:

Short Wheel Base arms with 996TT brakes

He had some brackets made up to bolt 996 calipers (which are nearly identical to the Boxster calipers I have) to the steel SWB control arms on his early 911,,,and they are gorgeous. Yes, I'm trying to get him to make me a set.
Old 08-30-2018, 05:36 PM
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Do not feel bad, your solution is very similar.
The only thing you need to do is finish machining the adapters you made.
Old 08-30-2018, 06:32 PM
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